This invention relates to a compass and, more particularly, to an electronic compass assembly and method for controlling the electronic compass assembly.
Electronic compass assemblies are often used in vehicles to indicate to a driver or passenger of the vehicle a direction that the vehicle is facing or traveling. A typical electronic compass assembly includes magnetic sensors that detect the magnetic field of the Earth. A microprocessor typically then determines the direction of the vehicle from the detected magnetic field and displays the direction as one of North, South, East, West, Northeast, Northwest, Southeast, and Southwest.
The magnetic conditions of the vehicle and surrounding environment of the vehicle typically change over time. As a result, the magnetic sensors are periodically calibrated to correct the magnetic field data for these magnetic changes. Calibration typically includes collecting magnetic field data from the magnetic sensors through a 360° turn of the vehicle (or a predetermined percentage of a 360° turn). The collected magnetic field data is generally ellipse-shaped. The microprocessor utilizes a statistical fitting procedure to produce a reference correction factor (e.g., a calibration ellipse) from the collected magnetic field data. The reference correction factor is then used to correct magnetic field data points before the microprocessor determines the vehicle direction.
The accuracy of typical electronic compass assemblies is susceptible to magnetic interference, sensor noise, and other conditions that affect the magnetic field data. In particular, nearby ferrous objects such as other vehicles, train tracks, or bridges to name a few examples may cause magnetic interference. Typically, the magnetically interfering object causes a change in the magnetic field that is detected by one or more of the magnetic sensors. The change in magnetic field skews the calculation of the vehicle direction by the microprocessor. This may result in a wrong direction being displayed to the driver or passenger without the driver or passenger being aware of the inaccuracy.
Typical electronic compass assemblies attempt to detect magnetic interference by filtering the magnetic field data points that are used to determine the compass headings. These methods often include comparing a collected magnetic field data point to previous magnetic field data points. If the collected magnetic field data point deviates a predetermined amount from the previous magnetic field data points, a magnetic interference condition exists. Another method includes averaging several successive magnetic field data points and comparing a subsequently collected magnetic field data point to the average to identify a magnetic interference condition. Disadvantageously, these methods may inaccurately identify magnetic interference conditions because too few data points are used. Using only a few data points, it is difficult to distinguish what are actually momentary magnetic disturbances from longer term magnetic interference. This often results in a wrong vehicle heading being displayed.
Accordingly, there is a need for an electronic compass assembly that identifies whether a magnetic interference condition exists in order to avoid displaying a wrong direction or to warn the driver or passenger that the displayed direction is unreliable. This invention addresses those needs and provides enhanced capabilities while avoiding the shortcomings and drawbacks of the prior art.